Data which has been transmitted over a channel, for example telephone line, microwave, satellite, or magnetic recording/playback channel may be distorted in the process of transmission such that adjacent data bits interfere with each other. As a result, even in the absence of noise or other disturbance, samples of the received signal taken with a clock synchronous with the data clock no longer correspond to the input data. This interference, generally referred to as intersymbol interference, may be linear or nonlinear, and will increase the likelihood of incorrect data decoding in the receiver.
According to well known signal detection practices, the received signal is sampled at the data clock rate, and the amplitude value of each obtained sample is compared to a threshold. When the amplitude value exceeds the threshold, it is detected as a binary one, otherwise it is detected as a binary zero. Because of the effect of the above-indicated channel transfer characteristics, the amplitude of the samples varies with the particular transmitted data pattern, due to intersymbol interference. Decision feedback decoders are known to be utilized to compensate for the intersymbol interference. In these decoders a known number of bits previously detected by the decoder, generally referred to as previous decisions, are fed back and stored. The known decision feedback decoders provide a correction value based on the previous decisions and particular channel characteristics, and adjust the threshold accordingly before making a next decision. The next decision is obtained by comparing the next received sample value to the adjusted threshold, thereby compensating for the intersymbol interference.
In applications where it is essential to transmit and decode data at a high rate, decision feedback decoders operating at a desired high speed are necessary. However, known decision feedback decoders are required to perform two basic operations in succession during each decision making step. These two sequential operations are: adjusting the threshold to compensate for the intersymbol interference, and comparing a newly received sample value to the adjusted threshold. Therefore, the operation speed of the decision making process is limited by a total delay obtained as a sum of operation delays effected by each circuit portion utilized for performing these sequential operations.